1. Field of the Invention
The invention disclosed herein relates to semiconductor devices using semiconductor elements.
2. Description of the Related Art
Nonvolatile semiconductor devices such as nonvolatile storage devices which are capable of writing and erasing data repeatedly, for example, an EEPROM and a flash memory are convenient and have high resistance to physical impact. Therefore, they have been mainly used for portable storage media such as a USB flash drive and a memory card, an RF tag which is a medium for radio frequency identification (RFID) in which data is read wirelessly, and the like, and have been widely available on the market. In the semiconductor device, a transistor which functions as a storage element is included in each memory cell. Further, the transistor includes an electrode called a floating gate, between a gate electrode and a semiconductor film serving as an active layer. The accumulation of electric charge in the floating gate enables storage of data.
Described in Patent Documents 1 and 2 is a thin film transistor including a floating gate which is formed over a glass substrate.